Generally, the paints designed for electrostatic spray coating are classified into a solvent type paint (an oil paint) which has a relatively large electric resistance, an aqueous type paint (a water paint) which has a relatively small electric resistance, and a metallic type paint with a dispersed metal powder content has a relatively small electric resistance similarly to the aqueous paint. Since the electric resistance of the paint varies depending upon the type or nature of the paint in this manner, it is the usual practice to apply high voltage by a different method for a different type of paint.
More specifically, the component parts such as paint tank and color changing valve are connected to the earth in operation from the standpoint of preventing danger. In this regard, in case of a solvent type paint with a relatively large electric resistance, there is no possibility of the rotary atomizer head being shorted to the earth potential through a paint supply conduit even if a high voltage is directly applied to the rotary atomizer head. Accordingly, the electrostatic spray coating machine for the solvent type paint is generally arranged to apply a high voltage directly to a rotary atomizer head for direct charging of paint particles.
On the other hand, in case of the aqueous type paint or metallic paint which has a small electric resistance, direct application of high voltage to a rotary atomizer head will invite the problem of shortcircuiting of the rotary atomizer head to the earth potential through the paint in the paint feed conduit, failing to charge the paint particles. Therefore, in case of an aqueous paint, it is the usual practice to apply a high voltage to external electrodes which are located in positions radially outward of a rotary atomizer head, forming a corona discharge region forward of the rotary atomizer head thereby to indirectly charge the sprayed paint particles from the atomizer head.
Illustrated in FIGS. 6 to 9 is an electrostatic coating machine of a prior art construction employing the indirect charging system for the aqueous paint.
In these figures, indicated at 1 is a coating machine of the rotary atomizer head type, which is largely constituted by a cylindrical housing 2 formed of a synthetic resin material (e.g., polytetrafluoroethylene), an air motor 3 mounted within the housing 2 and internally provided with an air bearing (not shown), a rotational shaft 4 rotationally driven from the air motor 3, a rotary atomizer head 5 mounted on the rotational shaft 4 on the front side of the housing 2, and a paint feed tube 6 in the form of a metal pipe passed through the rotational shaft 4 to supply paint to the rotary atomizer head 5. An insulate support 7 is projected on the rear side of the housing 2, the insulate support 7 being mounted on a reciprocator or the like. The basic construction of the air motor 3 is known from Applicant's prior application, Laid-Open Japanese Utility Model Application 60-13259, and therefore its detailed description is omitted here.
The reference numeral 8 denotes an annular electrodemounting bracket which is located radially on the outer side of the circumference of the housing 2 and radially outward of the rotary atomizer head. The bracket 8 is supported on the rear end of the housing 2 through support arms 8A. Indicated at 9 are supporters which are formed of a plastic material (e.g., polytetrafluoroethylene) to serve as electrode retainer rods which cover external electrodes 10 as will be described later. Each supporter 9 is provided with a recess 9A at its fore end, and six of this sort of supporters 9 are provided in equidistant positions around the circumference of the electrode brackets 8. As shown in FIG. 9, the external electrodes 10 are axially fitted and retained in the respective supporters 9, in such a way that the fore end 10A of each external electrode 10 is protruded out of the recess into a position flush with the fore end face of the supporter 9. Further, the fore end 10A of each external electrode 10 is located slightly rearward and radially outward of the rotary atomizer head 5.
Designated at 11 is a paint supply source, which is constituted by a motor 12, a paint pump 13, a paint tank 14 and so forth, storing an aqueous paint in the paint tank 14. The paint supply source 11 as a whole is connected to the earth 15.
Indicated at 16 is a pneumatically driven three-way change-over valve which is mounted on the insulate support 7, the inlet port of the three-way change-over valve 16 being connected to the paint pump 13 through a paint feed conduit 17 while the outlet port of the valve is connected to the feed tube 6 through a spiral hose 18 covered with a synthetic resin material. Further, the return port of the valve is opened into the paint tank 14 through a return conduit 19. The three-way change-over valve 16 normally communicates the paint feed conduit 17 with the return conduit 19 to relief the paint, and, when switched to an operating position, communicates the paint feed conduit 17 with the spiral hose 18 to supply the paint to the rotary atomizer head 5.
The reference 20 denotes a high voltage generator which is constituted, for example, by a Cockcroft circuit or the like, and electrically connected to the external electrodes 10 through a high voltage cable 21 to apply thereto a high voltage of -50.about.-90 kV. For this purpose, the fore end of the high voltage cable 21 is connected to the electrode bracket 8.
For coating a workpiece 22 by the use of the electrostatic spray coating machine of the construction as described above, the air motor 3 of the coating machine 1 is put in high speed rotation to drive the rotational shaft 4 and the rotary atomizer head 5 at a speed of 40,000.about.60,000 rpm. Concurrently, a high voltage is applied to the respective external electrodes 10 from the high voltage generator 20 through the cable 21 thereby forming a corona discharge region forward of the fore end 10A of the external electrodes 10. Further, the paint supply source 11 is actuated, but the aqueous paint is relieved through the three-way change-over valve 16. In this state, upon switching the three-way change-over valve 16 to the feed position, the aqueous paint in the tank 14 is fed to the rotary atomizer head 5 through the paint pump 13, paint feed conduit 17, three-way change-over valve 16, spiral hose 18 and feed tube 6. The paint is atomized by the rotary atomizer head 5 into micro particles, which are charged as they are passed through the corona region and caused to fly along the electrostatic field between the external electrodes 10 and the workpiece 22 for deposition on the latter.
In case of the above-described prior art electrostatic spray coating machine employing external electrodes, an aqueous paint is atomized by the high speed rotation of the rotary atomizer head 5 and sprayed in radial directions under the influence of centrifugal force. At this time, with aqueous paint, which uses water as a dispersant or diluent, the water content in the sprayed paint evaporates to increase the water density in the space between the rotary atomizer head 5 and the external electrodes 10. On the other hand, since a high voltage is applied to the external electrodes 10, the resulting water vapors are charged upon elevating the high voltage, increasing the value of current flow between the external electrodes 10 and the rotary atomizer head 5 to such a degree as to bring about the discharge phenomenon.
As a result of such a discharge phenomenon, the high voltage applied to the respective external electrodes 10 is shortcircuited to the earth 15, from the rotary atomizer head 5 through the feed tube 6, spiral hose 18 and three-way change-over valve 16. In order to prevent the discharge phenomenon of this sort, it is necessary to control the high voltage to be applied to the respective external electrodes 10, according to a maximum voltage VMAX which is automatically determined. For example, in a case where the distance (hereinafter referred to as "distance H") between each external electrode 10 and the rotary atomizer head 5 is 100 mm, the maximum voltage VMAX which can be applied to the external electrodes 10 free of the discharge phenomenon is predetermined to be in the range of -54.about.-57 kV, limiting the efficiency of paint deposition on the workpiece 22 to about 70.about.80%.
It has been known in the art that the deposition efficiency can be enhanced by increasing the voltage to be applied to the respective external electrodes 10. However, the prior art has a problem in that the paint deposition is rendered infeasible by the discharge phenomenon which takes place between the rotary atomizer head 5 and external electrodes 10, when a voltage in excess of the above-mentioned maximum voltage VMAX is applied to the external electrodes 10.
In order to solve this problem, it may be conceivable to increase the distance H of the external electrodes 10 from the rotary atomizer head 5. However, in such a case, although the maximum voltage VMAX can be increased, there will arise another problem that the number of the external electrodes 10 has to be increased because six external electrodes are not sufficient to make up for the reduction in density of the paint particles to be charged in the corona discharge region formed forward of the respective external electrodes 10. A further increase of the distance H will result in a lower velocity of the sprayed paint particles in the corona discharge regions, the sprayed paint particles being influenced by the static attractive force to a greater degree correspondingly to the reduction in velocity of the sprayed paint particles and attracted toward the external electrodes 10. Consequently, the external electrodes 10 are contaminated by paint deposition. Therefore, the external electrodes 10 should be located in positions where the distribution density and velocity of the sprayed paint particles are high enough to permit effective charging by a minimum number of electrodes.
Namely, since the initial velocity F of the sprayed paint from the rotary atomizer head and the static attractive force f in the corona discharge region are in the relationship of ##EQU1## it is advantageous to locate the respective external electrodes 10 at a distance H within a range which satisfies Equation (1) above, in order to charge the paint particles effectively by the use of a minimum number of external electrodes. Accordingly, even if the maximum voltage VMAX could be elevated, the method of simply locating the respective external electrodes 10 at a greater distance from the rotary atomizer head 5 is not the best way of enhancing the deposition efficiency.
In view of the above-mentioned problems of the prior art, the present invention has as its object the provision of an electrostatic coating machine with an external electrode system, which permits to elevate the high voltage to be applied to the external electrodes for enhancing the paint deposition efficiency without giving rise to the discharge phenomenon even in case of an aqueous or metallic paint.